Ferrous materials are widely used in building structures. Since they are readily corroded, various means have been employed to protect them from corrosion. Among these means, hot dip zinc plating or galvanizing is applied to a wide variety of ferrous materials ranging from small-sized joint members such as bolts to large-sized structural members such as H-shaped steels. However, a zinc coating formed by hot dip galvanizing does not have good resistance against corrosion or damage caused by salt which tends to occur in areas near the seashore, for example. Therefore, there was a need for a corrosion-preventing coating for ferrous materials which possesses improved corrosion resistance over a zinc coating.
Under the circumstances, it was found that hot dip Al--Zn alloy plating could produce a coating having outstandingly superior corrosion resistance compared to hot dip galvanizing. It was also confirmed that hot dip plating with an Al--Zn alloy containing about 55% Al, about 1.5% Si, and a balance of Zn was most suitable from the viewpoint of improvement not only in corrosion resistance of the coating itself but also in protection of the ferrous substrate by sacrificial corrosion of the coating. This Al--Zn alloy plating is now applied to a considerable proportion of mass-produced corrosion-preventing thin steel sheets.
In general, hot dip plating of a thin steel sheet is carried out in a continuous hot dip plating apparatus which comprises a continuous annealing unit and a hot dip plating tank which is located on the outlet side (downstream) of the continuous annealing unit. In a typical process using such a continuous hot dip plating apparatus, a steel sheet is initially heated in a non-oxidizing furnace kept in a very slightly oxidizing atmosphere for cleaning, and then is passed into a reducing furnace connected to the non-oxidizing furnace. In the reducing furnace, the steel sheet is subjected to reduction and annealing in a hydrogen-containing atmosphere. Subsequently, the steel sheet is introduced, without exposure to air, into a hot dip plating tank to apply hot dip coating thereto. Thus, the steel sheet is shielded from air throughout the process from the cleaning step to the entry into the hot dip plating tank, and degreasing of the steel sheet and reduction of an oxide layer (oxide scale or film) formed on the surface thereof are performed before the steel sheet is introduced into the hot dip plating tank. Therefore, hot dip plating of the steel sheet occurs under such conditions that it can be readily wetted by the molten metal in the plating tank. Although this type of continuous hot dip plating apparatus was developed for the purpose of galvanizing, it is also used to perform hot dip aluminum or Al--Zn alloy plating. Thus, hot dip Zn--Al alloy plating can be performed by utilizing the same equipment and system used for hot dip galvanizing, although it is necessary to modify the composition of the plating bath and the operating conditions accordingly.
In contrast, hot dip plating of ferrous materials other than thin steel sheets, for example, continuous hot dip plating of a steel wire, or batchwise hot dip plating of structural members or other various steel parts has been performed by dipping the steel material in a molten metal bath (plating bath) in air. In this case, even if the steel material is preliminarily degreased and pickled prior to plating, it is inevitably oxidized prior to entry into the plating bath. Therefore, a flux comprising one or more salts is applied to the steel material prior to plating in order to remove the oxide layer, which has been inevitably formed on the surface of the steel material, by fusion and thereby promote wetting of the steel material by the molten metal in the plating bath.
The flux can be applied either by a dry process or a wet process.
In the dry process, a steel material is treated with an aqueous solution of a flux and then dried such that the flux is deposited on the surface of the steel material. The steel material having the flux deposited thereon is thereafter dipped in a molten metal bath to perform hot dip plating.
In the wet process, a flux is placed onto a molten metal bath in a plating tank. The flux is fused by the high temperature of the molten metal bath and due to its lower specific gravity the fused flux floats on the molten metal bath. A bed of the fused flux having an appropriate thickness is formed onto the molten metal bath in this manner. When a steel material is introduced into the molten metal bath, it passes through the floating bed of the fused flux and is coated with the flux before entering the molten metal bath. In this case, when the steel material is withdrawn from the molten metal bath, it again passes through the floating bed of the fused flux such that the flux is deposited on the surface of the plated steel material. As a result, subsequent to hot dip plating, it is necessary to perform an additional step of removing the flux residues which remain deposited on the plated surface, thereby making the process complicated.
Flux treatment for hot dip galvanizing, for example, is usually performed by the dry process, which is simpler in operation, using an aqueous solution containing zinc chloride and ammonium chloride as a flux material. However, this flux cannot be used with a molten metal bath which contains aluminum, as employed in hot dip aluminizing (aluminum plating) or Al--Zn alloy plating, since aluminum in the molten metal bath reacts with a salt, primarily NH.sub.4 Cl, present in the flux to form readily subliming AlCl.sub.3, thereby causing the flux to decompose. As a result, the function of the flux is significantly damaged, thereby causing the formation of a number of bare (uncovered) spots in the resulting plated coating.
For this reason, flux treatment for hot dip aluminizing is normally performed by the wet process using a flux which comprises one or more fluoride salts. However, this flux has a relatively high melting point. Therefore, when it is used for hot dip Al--Zn alloy plating, it does not exhibit an adequate effect due to the lower melting point of the Al--Zn alloy compared to aluminum metal.
Several fluxes have been proposed which are suitable for use with hot dip Al--Zn alloy plating.
For example, Japanese Patent Application Laid-Open No. 58-136759(1983) discloses a flux composition for use with Al--Zn alloy plating which comprises zinc chloride and at least one additional salt selected from chlorides, fluorides, and silicofluorides of an alkali or alkaline earth metal. This flux is conveniently applied by the dry process. However, its function as a flux is not satisfactory. Namely, it tends to cause the occurrence of bare spots more frequently with increasing Al content in the molten metal bath. This phenomenon becomes striking particularly with 55% Al--Zn alloy plating, which has a high Al content and produces a highly corrosion-resistant coating.
Japanese Patent Application Laid-Open No. 3-162557(1991) discloses a flux composition for use with hot dip Al--Zn alloy plating which comprises zinc chloride and ammonium chloride at a weight ratio of from 10:1 to 30:1. This flux is also used by the dry process and it gives fairly good results in plating of thin sheets. However, the occurrence of bare spots increases as the plating temperature (temperature of the plating bath) increases. Therefore, in the case of 55% Al--Zn alloy plating in which the plating temperature is high, bare spots may often be formed in the resulting plated coating unless the ferrous material to be plated is a thin sheet.
Japanese Patent Application Laid-Open No. 4-293761(1992) discloses a flux composition for use with hot dip Al alloy plating which comprises chlorides salts of zinc, lithium, sodium, and potassium. The use of this flux is costly since it is applied by the wet process, and among the four chloride constituents, the most expensive lithium chloride comprises a major proportion (40-60%) of the flux. For plating of thick ferrous materials, its effect on prevention of the formation of bare spots is inadequate. In addition, hot dip plating must be followed by removal of the flux residues deposited on the plated surface.
Japanese Patent Application Laid-Open No. 4-323356(1992) discloses a flux composition for use with hot dip Al--Zn alloy plating which comprises an Al-containing alkali metal fluoride (e.g., cryolite) and an alkaline earth metal chloride. This flux is also used by the wet process and is disclosed as being particularly suitable for use in 55% Al--Zn alloy plating. However, it involves a problem that scaffolding of the flux (the phenomenon that the flux is solidified to make a shelf or scaffold and create a cavity between the molten metal and the solidified flux) tends to occur. Another problem is that since this flux contains a fluoride salt, the flux residues deposited and solidified on the plated surface during withdrawal of the plated steel material from the molten metal bath cannot be readily removed by rinsing with water or similar means due to the presence of the fluoride salt. As a result, the appearance of the plated surface becomes inferior.
Thus, when the conventional fluxes are used particularly for hot dip Al--Zn alloy plating having a relatively high Al content, i.e., on the order of 45% or higher, they cannot perform as a flux sufficiently by the dry process, and the formation of bare spots tends to occur frequently. When they are used by the wet process, the fluxes themselves may be expensive, or they may cause the scaffolding phenomenon, or removal of the flux residues deposited on the plated surface may be difficult, thereby causing the plated surface to have a deteriorated appearance.
Instead of using a flux, it is proposed to apply duplex hot dip plating to a steel material, i.e., by performing hot dip galvanizing followed by hot dip Al--Zn alloy plating, for example, in Japanese Patent Publication No. 61-201767(1986). However, this technique requires that a hot dip plating operation be performed twice, which is naturally disadvantageous from the viewpoint of manufacturing costs.
Furthermore, in a conventional hot dip Al--Zn alloy plating method, a preheating step, which can be performed prior to plating, is either totally eliminated or insufficiently performed. Therefore, the duration of dipping in the molten metal plating bath is as long as at least 20 seconds and usually from 30 seconds to 180 seconds. In particular, when the Al--Zn alloy contains from 45% to 60% Al, the temperature of the plating bath becomes high and hence a brittle intermetallic compound layer formed at the interface between the metal substrate and the plated coating (such layer being hereunder referred to as an "interfacial alloy layer") is caused to grow significantly during dipping in the plating bath, thereby adversely affecting the deformability or workability of the plated coating.
A plating tank which is used for hot dip Al--Zn alloy plating is normally made of a refractory material, a ceramic, or graphite, which is hard to corrode. Because of rapid corrosion, a ferrous material is not suitable as a material for such a plating tank. The shape of the plating tank is normally a rectangular box, since such a shape occupies a small space and receives a large volume of a molten metal bath. In a batchwise operation of hot dip plating, the molten metal bath in the plating tank is allowed to solidify when the operation is suspended for a long period, and it is heated to remelt the metal bath before the operation is resumed. Accordingly, solidification and melting of the metal bath are repeated in the plating tank. When the plating tank is made of a refractory material or the like, the inner wall of the plating tank tends to be cracked by the repeated solidification and melting. This significantly decreases the service life of the plating tank and may eventually cause leakage of the molten metal bath through the resulting cracks of the plating tank, which is very dangerous.